The only way Wales can get more money from the UK government is by voting yes in the referendum on assembly powers, says First Minister Carwyn Jones.
Speaking at the National Eisteddfod, Mr Jones said the UK government would see no need to change the way Wales was funded in the event of a no vote.
Mr Jones told BBC Wales: "If the outcome is a no vote then they are not going to address it. Why should they?"
A Treasury spokesman told BBC Wales that Mr Jones' fears were unfounded.
Mr Jones was speaking after setting out the case for a yes vote in next year's referendum on whether the Welsh assembly should gain more powers.
"The Conservative and Lib Dem government have said they won't consider the issue of Wales' funding unless they know the outcome of the referendum, to use the exact words," he said.
"Now thinking about our bottom dollar, if the outcome is a no vote, then they are not going to address it. Why should they?
"If there were to be a yes vote then they would then start to address that level of underfunding."
However Welsh Conservative MEP Kay Swinburne disagreed with Mr Jones' take on the situation.
She told BBC Wales: "Certainly before the election my party were already saying that it was time to re-think and re-work the formula for all the funding in all the parts of the UK.
"I think it's happening anyway no matter what the outcome of the referendum."
A Treasury spokesman said it was true that changing the way Wales is funded was not the UK government's priority, but that had nothing to do with referendums.
The spokesman added that the Treasury's current priority was to get to grips with the economy.